Method for preparing laminated film of polymeric substances

ABSTRACT

A method for preparing a laminated film of polymeric substances wherein each film layer constituting the laminate is closely surface bonded to each other, which method comprising heating, pressing and rolling between rollers a sandwich wherein both of the outer layers thereof are fluorocarbon resin films and at least one thermoplastic polymer is interposed therebetween as an inner layer or layers, said rollers being at a temperature not higher than the melting point of said fluorocarbon resin and not lower than 25*C.

United States Patent [1 1 Moriyama et a1.

[ METHOD FOR PREPARING LAMINATED FILM OF POLYMERIC SUBSTANCES {75] lnventors: Yasuhiro Moriyama; Takalumi Okamoto, both of Osaka, Japan [73] Assignee: Nitto Electric Industrial Co., Ltd.,

Osaka, Japan 22 Filed: Apr. 26,1971

21 Appl. No.: 137,558

[ Oct. 30, 1973 3,616,198 10/1971 Saito 156/309 X 3,657,038 4/1972 Lightt'oot 156/309 X 3,329,549 7/1967 Vilutis 156/324 X 3,486,961 12/1969 Adams... 156/324 X 3,666,587 5/1972 Nagao 156/309 X 3,676,289 7/1972 Hara et al. 156/306 X 3,677,845 7/1972 Roberts 156/306 X FOREIGN PATENTS OR APPLICATIONS 871,959 7/1961 Great Britain 156/309 Primary Examiner-Norman G. Torchin Assistant Examiner-John R. Miller, Jr.

Attorney-Sughrue, Rothwell, Mion, Zinn & Macpeak [57] ABSTRACT 19 Claims, 1 Drawing Figure [30] Foreign Application Priority Data Apr. 25, 1970 Japan ..45/35603 Oct. 8, 1970 Japan..... 45/88975 [52] I US. Cl. 156/309, 156/324 [51] Int. Cl. C09] 5/00, C09j 7/00 [58] Field of Search 156/309, 308, 306, 156/324 [56] References Cited UNITED STATES PATENTS 2,833,686 5/1968 Sandt [56/309 X 3,563,830 2/1971 Burgess..... 156/309 X 3,513,064 5/1970 Westley 156/309 X 3,411,965 11/1968 Hobaica 156/309 X PATENIEDUCTBOIQIS I 3.769.137

INVENTORS .RO MORIYAMA v I Ml OKAMOTO j g 1G4, fin w e w'h BY Z644 a uzcfeaK ATTORNEYS BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for preparing a laminated film of polymeric substances. More particularly, the present invention relates to a method for preparing a laminated film of polymeric substances having, as at least one of the two outer layers thereof, rolled fluorocarbon-resin films. I

2. Description of the Prior Art It has previously been well known to manufacture laminated products by effecting lamination on a press plate, under heating and pressure, or by using an extruder from which a polymeric substance is extruded through a T-die to give a film-formed material which is in turn superposed, using extrusion topping rollers, onto another film. With the former method, however, laminated products cannot be continuously obtained and, with the latter, the method generally is applied film, and more particularly 'to laminate on a film of a tetrafluoroethylene polymer, other thermoplastic polymer films, because of the remarkably poor surface energy of the fluorocarbon resin film which results in poor adhesiveness.

An object of the present invention is to provide a method for preparing a laminated film in which both of the outer layers are fluorocarbon resin films and the inner layer or layers are thermoplastic polymer films.

Another object of the present invention is to provide a method for preparing the above-described film laminates in a continuous manner.

A further object of the present invention is to provide the fluorocarbon resin film laminates which are superior in their electrical and mechanical properties.

SUMMARY OF THE INVENTION The present invention comprises, therefore, a method for manufacturing a laminated film of polymeric substances wherein each film layer constituting the laminated film is closely surface bonded to each kept at temperatures less than the melting point of a fluorocarbon resin and above 25C, a multilayer in which the outer layers are films of the fluorocarbon resin and at least one thermoplastic polymer film is interposed between the outer layers.

According to the present invention, there may be obtained laminated films of polymeric substances having high tensile strength and high dielectric breakdown voltage, in which each film is extremely closely surface bonded to each other.

BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWING The accompanying drawing shows an example of manufacturing devices employed in one embodiment of the method according to the present invention.

DESCRIPTION OF PREFERRED EMBODIMENTS OF THE INVENTION In the present invention, the rolling preferably may be effected to an extent such that the thickness of thealso improved, in proportion to the increase in draw ra-- tio. Whenthe-thickness becomes less than percent of that before the rolling, however, the mechanical strength of the laminate is lowered, so that, taking every aspect into consideration, the thickness most preferably ranges from 60 to 30 percent of the thickness before the rolling. The degree of pressing is dependent on the materials employed, their thickness, heating' temperatures and the like, although the degree of pressing is in general on the order of from about 20 to I 1,000 kg/cm. The rollers employed in the invention in general may be made of metals.

The fluorocarbon resin films employed according to the present invention include, by way of example, tetrafluoroethylene polymers, trifluoroethylene polymers, monochlorotrifluoroethylene polymers, copolymers between tetrafluoroethylene and hexfluoropropylene, vinylidene fluoride polymers, vinyl fluoride polymers and the like, although those films comprising the tetrafluoroethylene polymers, trifluoroethylene polymers or copolymers between tetrafluoroethylene and hexaacid and acrylic acid ester, polycarbonates, polyesters,

other by heating, pressing and rolling between rollers,

polyamides, polyamideimides, polyimides, polysulv fones, polyphenylene-oxides and the like, at least one of which is employed.

Furthermore, according to the present invention, there may be employed films of the polymers prepared by cross-linking polyethylene, chlorinated polyethylene having chlorine content of less than ,50 percent by weight, or polyethylene containing such a chlorinated polyethylenein any proportion of from0 to 100 percent by weight. In the case of such polymers, laminates may be obtained having, in addition to the various superior properties as described in the foregoing, good properties at high temperatures, in particular, high The cross-linking may be carried out using any of the previously known techniques as, for example, by electron ray irradiation or by chemical cross-linking. It is generally preferred to employ cross-linked polyethylenes of a gel fraction not more than about percent. The polymerization degree of polyethylene suitably is, in'general, on the order of about 1 to 5 X 10".

As the films of the present invention, any polymer capable of forming films, irrespective of the polymerization degree thereof both in the case of the outer layers and the inner layer or layers can be employed.

The'thickness of the films may be varied broadly, although extraordinarily large thickness is not desirable from a practical standpoint because too long a period of time is required for transmission of the heat of the metal rollers into the inner layer or layers.

The heating is effected between rollers kept at temperatures not less than 25C and not above the melting points of both of the outer fluorocarbon resin layers.

These melting points are, for example, about 327C in the case of tetrafluoroethylene polymers, from about 210 to 212C in the case of trifluoroethylene, about 285C in the case of copolymers between tetrafluoroethylene and hexafluoropropylene, about 170C in the case of vinylidene fluoride polymers and about 200C in the case of vinyl fluoride polymers.

Furthermore, on carrying out the heating as described above according to the present invention, when the temperatures of said rollers are lower than 25C, laminated films having a sufficiently high bonding strength between the layers will not be obtained. On the contrary, when the heating is carried out at roller temperatures above the melting points of the fluorocarbon resins, the outermost fluorocarbon resin film layers will be destroyed, thereby not resulting in laminated films. More specifically, the temperatures of the rollers are preferably not less than the heat distortion temperatures of the fluorocarbon resins and not more than their melting points. For example, in the case of tetrafluoroethylene polymers, the temperatures are not less than 120C and not more than 327C, although, in practice, those not more than 300C are preferred.

Reference will now be made to the accompanying drawing where an embodiment of the present invention is illustrated. Designations l, 2 and 3 are supply rollers of films and fluorocarbon resin films 4 and 4 are fed from the supply rollers 1 and 3, respectively, and a thermoplastic polymer film 5 from the supply roller 2. Designation 6 and 6 are heated rollers, between which heating, pressing, and rolling of the films 4, 4 and 5 are effected,

thereby giving a laminate 7 which is in turn guided through guide rollers 8 and 8 and then wound up by reeling roller 9.

in addition, it is desirable to treat the inside surface of the fluorocarbon resins employed as the outermost layers in the present invention using any known adhering treatment as for example by treating the same with a treating solution of sodium naphthalenetetrahydrofuran complex in tetrahydrofuran, or of sodium-liquid ammonia, thereby giving rise to a further enhancement in peel strength between the surface and a thermoplastic polymer film employed as the inner layer.

As described in the foregoing, one characteristic of the present invention herein described consists in a laminate manufactured using, as its outer polymer film layers, fluorocarbon resin films and we have found, to our surprise, that, as described above, by using the outermost layers composed of the fluorocarbon resin film, the rolling of the laminate can be readily effected due to its non-adhesivity and lubricating property, without oethylene polymers have high melting points, the rolling and lamination can be effected at any roll temperature above'the melting points or other thermoplastic polymer film or films as the inner layer or layers, with polymer films being of any type. Subsequently, in accordance with the presentinvention, it is possible to obtain a laminate having greater peel strength between the layers. I

As is evident from the foregoing, according to the present invention, a laminate can be obtained in a continuous manner and the kind, number and thickness of the inner polymer film layer or layers and, in the case of cross-linked polyethylenes, the cross-linking methods and degrees of gel fraction can be freely chosen, so that there may be obtained readily laminates having the characteristics desired.-The resulting laminates have sufficient peel strength between their layers to overcome the stress of use, to such an extent that they can be subjected to a second processing, such as cutting or punching. Furthermore, when the outermost fluorocarbon layer is rolled two times or more in the longitudinal direction, the tensile strength thereof is increased approximately two times and a marked improvement can be obtained in the dielectric breakdown voltage.

The, following examples are given for purposes of illustrating the present invention and are not to be considered as limiting the scope thereof.

EXAMPLE 1 300 g of a commercial moulding powder of tetrafluoroethylene (POLYFLON M-l2, mp. 327C, manufactured by Daikin Kogyo Co., Ltd), was filled in a cylindrical mould having an inner diameter of mm and then preformed under a pressure of 300 kglcm The resulting product was then, as such, sintered at a temperature of 360C for 3 hours followed by cooling slowly the resulting sintered product to room temperature at a rate of about 0.7C/min.-, thereby giving a cylindrical moulded product of about 48 mm thickness and about 62 mm length. The moulded product was then skived by a skiving method, thereby giving a film of 80p. thickness. Between two sheets'of the resulting film a low density polyethylene film (.SHOLEX F l2 1 having'a melting point of 1 10 and 120C and a molecular weight of 25 X 10, manufactured by Japan Olefin Chemicals Co., Ltd.), was interposed and the assembly was subjected to rolling between rollers of 20 em outside diameter kept at a temperature of 130C so that the thickness of the resulting laminate was 150, 100 and 75 41., thereby giving Laminates A, B and C, respectively. In this case, the roll pressure, i.e., the value calculated from the total load, applied between the rollers,

' 7 divided by the film width, was 34, 90 and 125 kg/cm.

any polymer film adhered onto the surface of the metal Subsequently, the above-described film of tetrafluoroethylene polymer was subjected to an adhesion treatment, on only one side surface thereof, in a conventional manner, thereby giving a Laminate D having, as in the above-described laminates, an inner layer of polyethylene film.

The various properties of the respective laminates are shown in the following Table 1.

TABLE 1 Laminate Laminate after Rolling before Properties Rolling A B C D Thickness of the Laminated Film (u) v 230 I50 I00 75 Thickness of the Tetrafluoroethylene Film (p) 8.0 60 40 30 75 Thickness of the Polyethylene Film (pt) 70 30 15 15 Feel Strength between the Tetrafluoroethylene Film and the Polyethylene Film 0 15 67 70 150 Dielectric Breakdown Average 6.8 7.2 11.2 1 1.8 12.8 Voltage (XV/0.1 mm) Minimum 4.5 5.7 9.5 9.7 9.4 Tensile Strength (in the direction of elongation) (kglmm 3.0 5.5 7.6 7.4 Elongation (in the direetion of elongation) 20.0 90 6O 60 The tensile strength, elongation and peel strength were all measured with an lnstron Tensil Tester at a crosshead velocity of 300 mm/min. The peel strength was estimated as the strength in g/ mm on peeling at 180 relative to the surface of the specimen. The dielectric breakdown voltage was measured according to A.S.T.M. D-149.

As is evident from the results shown in Table 1 above, Laminates B and C are superior in every respect to the Laminate A. In particular, Laminates B and C are markedly superior in peel strength to Laminate A. These results, after all, indicate that extremely good results can be obtained by setting the thickness of the roller and laminated product at less than 60 percent of the original thickness thereof. They also indicate that there can be obtained a marked improvement in peel strength between the layers by applying the adhesion treatment.

EXAMPLE 2 The cylindrically moulded product of the tetrafluoroethylene polymer employed in Example 1 was skived by a skiving method to produce a film of 50' p. thickness. Four sheets of the film were placed on the surface of each other and then rolled and laminated between the same rollers as employed in Example 1, using a roll temperature of 200C, in a manner such that thicknesses obtained after the rolling and lamination were the values indicated in Table 2 described hereinafter. In this case, the roll pressures between the rollers were as follows:

Laminate E l2 kg/cm Laminate F kg/cm Laminate G 52 kg/cm Laminate H 88 kg/cm Laminate l 120 kg/cm 1n the resulting'laminated films the thicknesses of which were not more than 60 percent of their original, the peel strength between the layers was 50 g/25 mm width. This strength is sufficient to employ the lamimore, about twice an improvement in their tensile strength was obtained also.

The characteristics obtained in the resulting laminated films which are rolled or unrolled are summa- As is evident from the results shown in Table 2 above, in laminates treated so that their thickness after rolling was. not more than 60 percent of their original thickness, a remarkable improvement in their electrical properties, as well as in the peel strength can be obtained.'Furthermore, it is to be noted also that, in this example, the defect inherent to tetrafluoroethylene polymer films obtained by skiving method, i.e., low electrical and mechanical strength due to their pinholes, were covered. Thus, the present example demonstrates that a multilayer tetrafluoroethylene polymer is integrated to cover the defect caused by pinholes and the integrated multilayer is then rolled to reduce the number of pinholes, thereby improving its mechanical properties.

EXAMPLE 3 Using as the outer layers two tetrafluoroethylene polymer films of the same type used in Example 2, each being of 50p. thickness, and, as the'inner layers, two low density polyethylene films of 501.1. thickness and of the same type as employed in Example 1 between which a polycarbonate film (UPILON having a melting point of 230C and a molecular weight of 3.2 X 10 manufactured by Mitsubishi-Edogawa Chemical Co., Ltd.), of 70p. thickness had been interposed, a laminate was manufactured by rolling and laminating the assembly at a roll temperature of 180C in a manner such that the laminate thus laminated was of l 10p. thickness. In the present example, there was employed a roll pressure of 70 kg./cm.

' The surface of the resulting laminate shows the properties of the tetrafluoroethylene resins, i.e., low friction factor, non-adhesivity and the like together with large rigidity and strength which'are not shown in the tetrafluoroethylene resins. In this example, the peel strength between the tetrafluoroethylene polymer and the polyethylene film was 70 g/25 mm. Various properties of the laminate anda tetrafluoroethylene polymer film of 110g thickness which is skived from the cylindrically moulded tetrafluoroethylene resin of the same type as employed in Example 1 are shown in the following Table 3.

TABLE 3 It is evident from the results shown in Table 3 above rized in the following Table 2. 0 that, as compared with the polytetrafluoroethylene TABLE 2 Laminate Laminate after rolling before Properties rolling E F G H 1 Thickness (I 200 180 150 120 100 Peel strength between layers (g/ZS mm)........ 0 0 7 46 53 56 Dielectric breakdown voltage (XV/0.1 mm) ...Average... 8.0 9.1 9.8 12.5 13.6 13.5 Minimum... 7.1 8.0 8.5 10.6 11.4 11.7

Dielectric breakdown voltage in unlaminated and untreated laminate (XV/0.1 mm) ..Average... 6.7 7.1 7.0 7.4 7.4 7.5 Minimum... 4.0 4.6 4.7 4.5 4.2 4.1

film, the laminate is markedly improved in its tensile elasticity and tensile strength.

EXAMPLE 4 Between two films of a tetrafluoroethylene-hexafluoroethylene cop'olymer ("TEFLON FEP, hexafluoropropylene content 15-16 wt percent, m.p. 285C, manufactured by E. l. Du Pont de Nemours), each being of 100 ,1. thickness, was interposed a tetrafluoroethylene'polyme'r film of the same type as employed in Example 3, and the assemblywas laminated by heating and rolling the same between the same metal rollers as employed in Example 3 which were kept at a temperature of 120C, in a manner such that the rusulting laminate was of 125 1.. 1n the present example, there was employed a roll pressure of 220 kg/cm. The thickness of the outermost layers was 50 pm, and the peel strength between the layers was 40 g/25 mm. The resulting laminate is also characterized as having heat seal properties.

EXAMPLE 5 One side surface of the film of 50 u thickness manufactured by extrusion moulding of DYFLON M- 3001"" (polytrifluoroethylene having a melting point of 212C and molecular weight of 8.7 X manufactured by Daikin Kogyo Co., Ltd.), was subjected to an adhesion treatment using the same sodium complex salt as employedin Example 1. Between these. two films where the treated surface of each was in a face-to-face position was interposed a film of 50p thickness prepared by extrusion moulding of TORAYs MOULD- lNG NYLON CM 1031 (polyamide ofnylon 6 type, mp. 215C, manufactured by Toray Industries, lnc.), followed by rolling between two rollers kept at a temperature of 140C in a manner such that the thickness of the resulting laminate was of 75 p thickness. In this case, the roll pressure between the rollers was 120 kg/cm. According to this procedure, there was obtained a laminate superior in its mechanical and electrical properties.

EXAMPLE 6 H Polyvinylidene fluoride (Kynar No. 300 having a melting point of 170C and a molecular weight of 5 X 10 manufactured by Pennsalt Chem. Co.), was extrusion moulded to obtain a film of 100 4:. thickness, one side surface of which was then subjected to an adhesion treatment using the same sodium complex salt as employed in Example 1. Between these two films in which the treated surface of each was in a face-to-face position was interposed a diacetylcellulose film (Acetylol' Sheet VRR-124", having a softening point of 91C,

EXAMPLE 7 Between two skived polytetrafluoroethylene (m.'p. 327C), films of 50; thickness was interposed a crosslinked polyethylene of 100 p. thickness ("lRAX FILM manufactured by Sumitomo Denko 104., crosslinked by electron beam irradiation, gel fraction 44 percent,

- pie 4, each being of 60 3:. thickness, was interposed a v molecular weight 2 X 10"), and the assembly was passed between two metal rollers having an outer diameter of 20 cm and kept at a temperature of 150C where it was heated, rolled and laminated to obtain polymer Laminates J, K or L of 130, 120 or 100 p, thickness, respectively. The roll pressures between the rollers were 40, 50 and 60 kg/cm, respectively. The various characteristics of each laminate are shown in the following Table 4.

. The tensile. strength, elongation and peel strength were all measured with an lnstron Tensil Tester at a cro'sshead velocity-of 300 mm/min. Thepee l strengths were estimated as the length in g/25 cm on peeling at 180 relative to the surface of the specimen. The di- As is evident from the results shown in Table 4 above, Laminates K and L are superior in every respect to Laminate J. ln particular, Laminates B and C are markedly superior in peel strength to Laminate A. These results, after all, demonstate the fact that, when the thickness of the laminate is lowered by rolling to not more than 60 percent of its original thickness, there may be obtained extremely preferred results.

' Furthermore, when eitherof the two polytetrafluoroethylene films is stripped from the laminate thus obtained, the resulting laminate possesses heat seal properties.

EXAMPLE 8 Following the procedure described-in Example 7 but using a cross-linked polyethylene film of p. thickness obtained by'admixing, with the cross-linked polyethylene film, a chlorinated polyethylene (chlorine content 35 wt percent) in an amount of 30 percent by weight,

there was obtained a laminate of p. thickness. The

' roll'pressure on pressing was 70 kg/cm. The characteristics of the resulting laminate were almost the same as in the laminate obtained in Example 7, which include. unlike the latter, firing resistance.

EXAMPLE 9 Between two films of the same tetrafluoroethylenehexafluoroethylene' copolymer as employed in Examcross-linked polyethylene of the same type as employed in Example 7 and of 50 p. thickness. The assembly was then rolled and laminated, as in Example 7, using a roller temperature of C and roller pressure of 70 kg/cm, thereby giving a laminate of 70 thickness. The

peel strength between the layers of the resulting laminate was 42 g/25 cm.

Although the invention has been described in considerable detail with reference to certain preferred embodiments thereof, it will be understood that variations and modifications can be effected without departing from the spirit and scope of the invention as described hereinabove and as defined in the appended claims.

What is claimed is:

1. A method of forming a laminate having a high tensile strength and dielectric breakdown voltage, said laminate consisting essentially of outer layers of a fluorocarbon resin film and, interposed between said outer layers, at least one film layer of a film-forming thermoplastic resin, each layer of said laminate being strongly surface-bonded to each adjacent layer thereof to provide a high peel strength between said adjacent layers, said method comprising:

l. forming a laminate of an outer layer of a fluorocarbon resin film, an intermediate layer of at least one film of a film-forming thermoplastic resin and an outer layer of a.fluorocarbon resin film; and

2. passing said laminate through a pair of compression rollers heated to a temperature of from 25C to the melting point of said fluorocarbon resin to simultaneously heat, compress and roll-draw said laminate thereby strongly surface-bonding said adjacent layers to each other and simultaneously reducing the thickness of both of said outer layers to a thickness equal to from 30 to 60 percent of their thicknesses prior to said rolling.

2. The method according to claim 1 further comprising after said step (2), stripping from the resulting laminate one of said outer layers of fluorocarbon resin film to provide a laminated film having heat sealing properties.

3. The method according to claim 1, wherein the heating is effected at a temperature not more than the melting point of said fluorocarbon resin constituting the outer layers and not less than the heat distortion temperature thereof.

4. The method according to claim 1, wherein the heating is effected at a temperature not more than the melting point of said fluorocarbon resin constituting the outer layers and not less than the melting point of said film-forming thermoplastic resin constituting the inner layer or layers.

5. The method according to claim 1, wherein said fluorocarbon resin is a tetrafluoroethylene polymer.

6. The method according to claim 1, wherein said fluorocarbon resin is a copolymer of tetrafluoroethylene and hexafluoropropylene.

7. The method according to claim 1, wherein said fluorocarbon resin is a trifluoroethylene polymer.

8. The method according to claim 1, wherein said fluorocarbon resin is a vinylidene fluoride polymer.

9. The method according to claim 1, wherein the surfaces of the outer layers of the fluorocarbon resin films adjacent to said inner layer or layers are adhesiontreated.

10. The method according to claim 9 wherein said adhesion treatment comprises treating said surfaces with a solution of a sodium naphthalenetetrahydrofuran complex in tetrahydrofuran or with a solution of sodium-liquid ammonia.

11. The method according to claim 1 wherein said film-forming thermoplastic resin is selected from the group consisting of polypropylene, polyvinyl chloride, polyvinyl acetate, polyvinylidene chloride, polyvinyl alcohol, vinyl acetate-ethylene copolymers, acrylic acid-acrylic acid ester copolymers, polyesters, polyamideimides, polyimides, polysulfones, polyphenylene oxides and mixtures thereof.

12. The method according to claim 1, wherein said film-forming thermoplastic resin of the inner layer or layers is a fluorocarbon resin.

13. The method according to claim 1, wherein said film-forming thermoplastic resin of the inner layer or layers is a polyethylene.

14. The method according to claim 1, wherein said filmforming thermoplastic resin of the inner layer or layers is a polycarbonate.

15. The method accordingto claim 1, wherein said film-forming thermoplastic resin of the inner layer or layers is a tetrafluoroethylene polymer.

16. The method according to claim 1, wherein said film-forming thermoplastic resin of the inner layer or layers is a polyamide.

17. The method according to claim 1, wherein said film-forming thermoplastic resin of the inner layer or layers is a diacetylcellulose.

18. The method according to claim 1, wherein said film-forming thermoplastic resin is obtained by crosslinking a polyethylene containing from 0 to 100 wt percent of chlorinated polyethylene with a chlorine content of not more than 50 percent by weight.

19. The method according to claim 18, wherein said crosslinked polyethylene has a gel fraction not more,

than percent. 

2. passing said laminate through a pair of compression rollers heated to a temperature of from 25* C to the melting point of said fluorocarbon resin to simultaneously heat, compress and roll-draw said laminate thereby strongly surface-bonding said adjacent layers to each other and simultaneously reducing the thickness of both of said outer layers to a thickness equal to from 30 to 60 percent of their thicknesses prior to said rolling.
 2. The method according to claim 1 further comprising after said step (2), stripping from the resulting laminate one of said outer layers of fluorocarbon resin film to provide a laminated film having heat sealing properties.
 3. The method according to claim 1, wherein the heating is effected at a temperature not more than the melting point of said fluorocarbon resin constituting the outer layers and not less than the heat distortion temperature thereof.
 4. The method according to claim 1, wherein the heating is effected at a temperature not more than the melting point of said fluorocarbon resin constituting the outer layers and not less than the melting point of said film-forming thermoplastic resin constituting the inner layer or layers.
 5. The method according to claim 1, wherein said fluorocarbon resin is a tetrafluoroethylene polymer.
 6. The method according to claim 1, wherein said fluorocarbon resin is a copolymer of tetrafluoroethylene and hexafluoropropylene.
 7. The method according to claim 1, wherein said fluorocarbon resin is a trifluoroethylene polymer.
 8. The method according to claim 1, wherein said fluorocarbon resin is a vinylidene fluoride polymer.
 9. The method according to claim 1, wherein the surfaces of the outer layers of the fluorocarbon resin films adjacent to said inner layer or layers are adhesion-treated.
 10. The method according to claim 9 wherein said adhesion treatment comprises treating said surfaces with a solution of a sodium naphthalene-tetrahydrofuran complex in tetrahydrofuran or with a solution of sodium-liquid ammonia.
 11. The method according to claim 1 wherein said film-forming thermoplastic resin is selected from the group consisting of polypropylene, polyvinyl chloride, polyvinyl acetate, polyvinylidene chloride, polyvinyl alcohol, vinyl acetate-ethylene copolymers, acrylic acid-acrylic acid ester copolymers, polyesters, polyamideimides, polyimides, polysulfones, polyphenylene oxides and mixtures thereof.
 12. The method according to claim 1, wherein said film-forming thermoplastic resiN of the inner layer or layers is a fluorocarbon resin.
 13. The method according to claim 1, wherein said film-forming thermoplastic resin of the inner layer or layers is a polyethylene.
 14. The method according to claim 1, wherein said film-forming thermoplastic resin of the inner layer or layers is a polycarbonate.
 15. The method according to claim 1, wherein said film-forming thermoplastic resin of the inner layer or layers is a tetrafluoroethylene polymer.
 16. The method according to claim 1, wherein said film-forming thermoplastic resin of the inner layer or layers is a polyamide.
 17. The method according to claim 1, wherein said film-forming thermoplastic resin of the inner layer or layers is a diacetylcellulose.
 18. The method according to claim 1, wherein said film-forming thermoplastic resin is obtained by cross-linking a polyethylene containing from 0 to 100 wt percent of chlorinated polyethylene with a chlorine content of not more than 50 percent by weight.
 19. The method according to claim 18, wherein said crosslinked polyethylene has a gel fraction not more than 90 percent. 